Printing of multi-layer circuits

ABSTRACT

A sheet-fed system designed to print multilayer PCBs is introduced. The system consists of four main blocks; a drilling station, a patterning station, a stacking/bonding station, and a sintering zone. The substrate PCB is shuttled between these various stations, to have vias drilled, to be attached to stacks of previously-processed layers, to be covered with conductive paths by means of the aforementioned ink, and to have the ink sintered under a controlled temperature and atmosphere. Patterning is accomplished by means of a novel two-step method involving both high-temperature conductive elements, low-temperature conductive elements, and flux. Two such compositions are successively applied and individually sintered to form a single conductive path; the second application serves to fill the porosities of the first layer. By this method, a highly-conductive trace is obtained without requiring high temperatures, which in turn allows use of common substrates including polymers.

This application claims priority from U.S. provisional patent 64/146,452filed 13 Apr. 2015.

FIELD OF THE INVENTION

The present invention relates generally to the field of multilayerprinted circuit boards.

BACKGROUND OF THE INVENTION

Printed circuit boards (hereinafter, PCBs) are the backbone of almostevery electronic device. Modern consumer electronics, the automotiveindustry, medical devices, and industrial equipment of all varieties arecontrolled to an increasing degree by electronic circuits, which in turnrequire development of tens of thousands of new boards every year. Thistrend will grow as time-to-market and innovation have become majorcompetitive advantages. Despite this trend, to date there are currentlyno efficient tools to assist production of reliable PCB prototypes whichare required during the validation stages of a new product. In thecourse of product development, a board designer will typically order 2-3prototype versions to validate and test the electronic performance of adevice. Such prototypes are usually produced either by localsmall-medium PCB manufacturers or by Far East manufacturers (mainlyChina). They offer relatively fast delivery depending on the boardcomplexity, which is mainly related to the number of layers and thecircuit density. Such prototypes can be very expensive and may take afew weeks to manufacture. Several attempts to deliver prototypemanufacturing solutions such as CNC or Laser based machinery foraccurate etching of copper have proved unsuccessful mainly due to theircomplexity and it to reliably simulate “real” PCBs. Therefore thesemethods have not been widely adopted the industry.

The rapid evolution of 3D printing and additive manufacturing systemsduring the last few years raises new opportunities. It is more thanreasonable to predict that these technologies will produce someinnovative solutions in the realm of PCB and printed electronicsmanufacturing in the next few years. A few 3D printers featuringconductive inks for simple electronics applications have alreadyappeared but a system that can produce a commercial multilayer PCBprototype using some type of 3D printing technology is still lacking,and would fulfill a long-felt need.

SUMMARY OF THE INVENTION

We introduce a sheet-fed system designed to print multilayer PCBs usinga novel method. The system consists of four main blocks; a drillingstation, a patterning station, a stacking/bonding station, and asintering zone. The substrate PCB is shuttled between these variousstations, to have vias drilled, to be attached to stacks ofpreviously-processed layers, to be covered with conductive paths bymeans of the aforementioned ink, and to have the ink sintered under acontrolled temperature and atmosphere, respectively.

The patterning is accomplished by means of a novel two-step methodinvolving both high-temperature conductive elements, low-temperatureconductive elements, and flux. Two such compositions are successivelyapplied and individually sintered to form a single conductive path; thesecond application serves to fill the porosities of the first layer. Bythis method, a highly-conductive trace is obtained without requiringhigh temperatures, which in turn allows use of common substratesincluding polymers.

The foregoing embodiments of the invention have been described andillustrated in conjunction with systems and methods thereof, which aremeant to be merely illustrative, and not limiting. Furthermore just asevery particular reference may embody particular methods/systems, yetnot require such, ultimately such teaching is meant for all expressionsnotwithstanding the use of particular embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments and features of the present invention are described hereinin conjunction with the following drawings:

FIGS. 1,2 depict the main system elements and fabrication process flow.

FIG. 3 shows the conductive ink process.

FIG. 4 shows a printed stack of 3 layers (cross section)

FIG. 5 shows a treated sheet cross section and drilled sheet inreverse-side view.

DEFINITIONS

Hereinafter, the term ‘via’, or ‘vertical interconnect access’, is anelectrical connection between layers in a multi-layer PCB, which travelsthrough the plane of one or more adjacent layers.

The term ‘blind via’ is used to denote a via used to connect an outerPCB layer with at least one inner layer of a multi-layer PCB.

The term ‘buried via’ refers to a via connecting at least two innerlayers of a multi-layer PCB.

The term through-hole refers to a via around a hole passing through alllayers of a multilayer PCB.

The term ‘trace’ or ‘signal trace’ refers to a conducting pathconsisting of a flat, narrow conductive path.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention will be understood from the following detaileddescription of preferred embodiments, which are meant to be descriptiveand not limiting. For the sake of brevity, some well-known features,methods, systems, procedures, components, circuits, and so on, are notdescribed in detail.

The inventive system is a sheet-fed system designed to print multilayerPCBs using a novel conductive ink. This ink is printed onto standardcommercial thin insulator sheets which are widely used in the PCBindustry, such as Epoxy, FR4, Polyamides, Kapton and others.

The system consists of several ‘stations’ each performing a certainoperation, in the manner of an automated assembly line. A drillingstation, a stacking unit, a patterning station and a sintering zone areillustrated schematically in FIG. 1.

The feeder 101 containing multiple single-layer sheets feeds out asingle PCB substrate layer 102, using roller 103 to pay out the newsingle PCB layer.

The drilling station 104 is used to form holes through single ormultiple PCB layers 102 and thus allows for creation of vias.

The drilling station 104 is fed, either manually or automatically, witha single sheet per layer. This sheet is aligned (mechanically,optically, or otherwise) and vias (at this point simply holes, until thesheet is attached to other layers) are drilled according to theircoordinates in a Gerber (or any other format) file of the layer.

Different drill diameters are available in order to allow for variousvia dimensions. An automatic optical inspection camera inspects thesurface of the sheet upon conclusion of the drilling step, to verify thevias' integrity.

In some embodiments of the invention, the mechanical drilling apparatusmay be replaced by a laser drill to allow faster performance as well assmaller via dimensions.

The stacking unit 109 aligns and attach a drilled substrate 102 on themultilayer PCB substrate 110. The multilayer PCB substrate 110 is formedfrom several individual layers such as 102, glued or otherwise attachedto one another.

The conductor patterning station has two individual conductor componentdispensers 105, 106 adapted to form highly conductive traces asdescribed above, namely by dispensing a first layer (e.g. from dispenser105), sintering it (at the next station), and then laying down a secondlayer (e.g. from dispenser 106) that largely fills the porosities of thefirst layer.

The sintering station 107 allows for partially melting the conductivecomponents laid down by the patterning station.

The substrate layer 102 and multilayered PCB 110 are shuttled betweenthese various stations: drilled, stacked successively, covered withconductive paths, and sintered to form multilayer PCBs 110.

The transport of the substrate between processing stations is shownschematically in FIG. 2a-i , where the successive positions of thesingle substrate layer 102 are shown in representative individual stepsa-i. First an individual PCB layer sheet 102 is extruded or rolled froma cartridge into position, FIG. 2a . After the sheet is ready andpositioned, it is moved to the drilling station as in FIG. 2b , where itis aligned (registered) and drilled. When the sheet is ready with allthe vias drilled and is validated,

The single sheet 102 is aligned with previously-processed layers 110(assuming there are such), in one embodiment making use of the drillpattern either mechanically or by an automatic optical system (or both)to ensure tight registration. Except for the case of the very firstsheet, the single sheets 102 can be glued on top of previous finishedlayers of the processed PCB 110 either by means of thin, hightemperature-compatible adhesive film (such as 3M VHB Adhesive TransferTape) or by means of hot-press to a prepreg intermediate layer. Pressureis applied mechanically with a roller 109 or by any other means. Theroller can be either cold or hot. This gluing step may be accomplishedat the same station as one of the other steps, or at a dedicatedstation. In one embodiment of the invention, the gluing can be done intwo steps. In such embodiments, a first ‘soft’ gluing is performedduring the layering of the PCB stack 110, and a final ‘hard’ gluing isperformed after all layers are stacked.

It is within provision of the invention that a dedicated gluing stationbe employed to bind the new top layer 102 to the extant processed layers110, as shown in FIGS. 1,2.

The conductive route patterning devices, (which typically comprise adispenser, inkjet head or any other suitable kind of injection device)then applies the conductive ink according to the patterning process tobe described in detail below. This step includes deposition of traces,vias, pads etc. The printing algorithm can be line by line (raster),vector, or any other method depending on the device mobility (i.e.whether the patterning device is mounted on a scanner, articulated armor other motion assembly).

The inventive process involves deposition of two conductive inks,overlaid in two steps. Thus each ink (in some embodiments) has its ownnozzle and cartridge, with the deposited layer paths being aligned towithin a fine tolerance. Alternatively, these two steps may beconsolidated into one step, for instance with different compositionsbeing extruded from a single printhead, or by use of two passes of thesame composition extruded from a single printhead.

After deposition of all the conductive features of a layer as in FIG. 2d, the conductive ink is sintered at the sintering station 107 as in 2 e.Then the layer is returned to the patterning station 105,106 for thesecond layer of conductive ink to be deposited, as in FIG. 2f .Subsequently it is returned for a second sintering step, FIG. 2 g.

The next sheet is dispensed (FIG. 2h ), drilled (FIG. 2i ) and then asbefore is aligned and attached on top of the already-processed PCB stackby means of a thin adhesive material or by a thin prepreg layer. Then,the next conductive features are printed at the patterning stationaccording to the layout of the next sheet. This process is repeated Ntimes (for a multilayer PCB of N layers) until the last (top) layer isfully printed and sintered.

When the last layer has been stacked, printed and sintered, anadditional layer after drilling is attached, acting as a solder mask forthe component assembly stage. Upon completion, the entire stack istransferred back to the drilling station and the board is cut to itsfinal external dimensions.

Additional finishing steps such as silk printing, legend printing andsolder mask printing can be further incorporated either offline orin-line with this system.

Conductive Ink and Patterning Process

The conductive ink is made by depositing two materials, hereinafterreferred to as Ink A and Ink B—one on top of the other, in two steps, asdescribed above and as shown in detail in FIG. 3. The rationale behindthis process is to enable a film that is at once thin, highlyconductive, and which does not require any high-temperature processing.

Ink A determines the physical dimensions of the layer (namely the widthand thickness, as well as a degree of conductivity), and theseparameters can be controlled to an extent as will be familiar to oneskilled in the art of 3D printing. Ink B provides the high conductivityproperties while maintaining the structure defined by Ink A. As anexample, the resistivity after the first sintering step may be 20-50uOhm-cm, and after sintering of the 2^(nd) layer it has decreased toapprox. 10 uOhm-cm. This surprising result will be explained below.

Ink A is composed of a mixture of a high melting point metallic powder,such as copper, silver, gold etc., and a low melting point metallicpowder, such as zinc, tin, lead, alloys of such metals, etc., plus aflux paste, either organic, inorganic, or a mixture thereof. The ratioof high melting point powder to low melting point powder may varybetween 5:1 and 1:5, while the flux content may be 10%-20% by weight.

Ink B is a mixture low melting point metallic powder, such as commercialsolder alloys etc., and flux paste at an appropriate ratio of 10%-20% byweight.

Sintering powders made of materials with high electrical conductivity,such as copper or silver, on a polymeric substrate is impossible due totemperature limitations of the polymer, which will decompose atrelatively low temperature. Lowering the sintering temperature can beachieved by mixing such powder with a low melting point powder such astin alloy. However, when such sintering is done without applyingpressure, the layers obtained are highly porous and their conductivityis poor. In order to lower the porosity to a minimum, the second step ofthe invention is applied. Densification is obtained in a capillary flowmanner by melting a metal with a low melting point, such as solderalloy, over the sintered porous first layer. As in other solderingprocesses, use of flux is desirable to ensure the wetting of the twomaterials. The second layer effectively ‘fills the gaps’ left in thefirst layer, thus achieving a far higher level of conductivity withoutrequiring high sintering temperatures.

The conductivity of the printed pattern can be estimated as the weightedaverage of the conductive metal/alloys involved, reduced by theresistances of the mating surfaces.

The patterning process is shown in FIG. 3a-d . The patterning processproceeds as follows: a first layer is printed with Ink A (by writingwith head A of FIGS. 1,2) on the surface of the substrate film (FIG. 3a). These traces are then sintered by means of an external heatingsource, such as a radiative heat source (focused IR, halogen lamp, laserbeam, etc.). On hardening, Ink A transforms into a somewhat conductive,porous film as in FIG. 3 b.

Then, a second layer composed of Ink B is applied directly on top of inkA by printing head B, as in FIG. 3c . Upon heating, Ink B melts and isabsorbed into the pores of film A to form a more fully solid layer withouter dimensions determined by ink A (very similar to the way a spongeabsorbs water), as shown in FIG. 3 d.

The dimensions of the final conductive layer (A+B), as well as viafilling, are largely determined by the outer dimensions of layer A solong as liquid B does not overfill the pores of layer A.

The ratio between materials A and B on printing is in a range of1/5-5/1; a ratio of 1/1 is a reasonable example.

The second layer serves to fill the open porosities left after sinteringof the first layer, thereby dramatically increasing the conductivity ofthe resulting layer without requiring high sintering temperatures.

The Stacking Process

As described above, after the drilling processes is completed, eachsheet is aligned and stacked on top of its preceding (already printed)layer. The final result of this process is shown in detail in the crosssection shown in FIG. 4. Printing of a layer begins only after it isstacked upon the preceding layers. Here the bottom PCB layer 407 has hadconductive ink traces 408 deposited on it as described above. Anadhesive layer 406 was then glued on top and a second layer 405 attachedonto the first layer, after vias such as that shown at 410 were drilledat the drilling station. Conductive ink paths 401 are then deposited andsintered in turn onto the second layer 405 as described above. Onceagain an adhesive layer 404 is deposited onto the middle PCB layer 405and a top PCB layer 403 attached, after having been drilled with vias402; it is then printed with conducting ink and the multilayer PCB isready for use.

Vias 411 passing through the entire stack may be drilled after assemblyof all the layers.

In order to promote good adhesion between two adjacent layers during theentire manufacturing process and during the reflow process thereafter,by which electrical components are soldered to the PCB, a hightemperature adhesive is used, such as a 3M VHB Adhesive Transfer Tape orothers. In one embodiment, a thin adhesive layer protected by a Mylarfilm is applied to the reverse side of each insulator sheet reversesides. Another alternative for layer adhesion is the use of apre-impregnated glass-epoxy intermediate layer called prepreg, that canbe placed between 2 adjacent layers and then pressed in the presence ofheat. In FIG. 5 a series of registration holes and release cuts in thetreated sheet are shown. These allow alignment of the sheet to theprinting system coordinates and easy removal of the Mylar just beforethe sheet is attached to the sheet below it in the stack.

Such a sheet, with a reverse-side Mylar film and an enclosed adhesive,allows for easy handling during drilling of the vias, protects theadhesive layer, and allows easy alignment to the preceding layer beforeand after removal. The new sheet is attached to the stack in such a wayas to ensure smooth, wrinkle-less adhesion. In case a thicker insulatinglayer is required, multiple sheets can be drilled and then stacked,while only the last one is printed. This process can be performedmanually or automatically.

Consumable parts in the system include:

1. The ink cartridges (inks A and B)

2. The sheets (various materials, with adhesive, Mylar, holes andrelease cuts)

Board Finishing

In the traditional multilayer PCB process, a bare board undergoesseveral finishing processes to ensure successful component placement andattachment via solder reflow. The main process steps involved are:

1. Solder masking: deposition of an insulator material to ensure thatsolder drops are confined and don't create a short between two adjacentpads. This also prevents contamination of the top surface.

2. Gold or Silver coating on the exposed pads to ensure goodsolder-ability to the underlying copper (which oxidizes otherwise).

3. Solder paste printing: placing solder paste drops on the pads.

4. Component placement according to the layout—usually done by a robot.

5. Reflow furnace soldering: hot air oven with a defined temperatureprofile (according to the solder used and loaded board heat capacity) toensure that the components are solidly connected to the board.

With the exception of step 4, the system in accordance with anembodiment of the invention is able to perform all of these steps, orequivalent ones, to allow the creation of a fully assembled board. It isfurther within provision of the invention to employ a pick-and-placerobot to perform step 4 and thereby allow for a full end-to-end PCBprototyping device.

1. Solder masking: instead of insulator deposition, a thin insulatingsheet can be placed on top of the upper layer, in the same way asdescribed for our stacking process (see FIG. 5).

2. The conductive material described (FIG. 3) is not oxidation prone andcontains a significant volume of solder, which enables very goodadhesion to the solder in the soldering step. This may eliminate theneed for gold or silver coating when using our system.

3. Solder paste printing: our system is capable of depositing solder indiscrete locations as described in the System section.

4. Placement: requires dedicated equipment which may be incorporated inthe invention.

5. Reflow: the system is capable of creating a well-defined temperatureprofile using the heater used for the sintering process (FIG. 1).

It is within provision of the invention that the PCB can be madeflexible. This ultimately depends on the type of adhesive used. Acrylicadhesive will give a flexible PCB while prepreg adhesion will result ina rigid one.

Such flexible PCBs will find use in a number of applications such asflexible displays, wearable electronics, hinged or otherwise movableparts, energy harvesting devices, and others.

The foregoing description and illustrations of the embodiments of theinvention has been presented for the purposes of illustration. It is notintended to be exhaustive or to limit the invention to the abovedescription in any form.

Any term that has been defined above and used in the claims, should beinterpreted according to this definition.

The reference numbers in the claims are not a part of the claims, butrather used for facilitating the reading thereof. These referencenumbers should not be interpreted as limiting the claims in any form.

1-10. (canceled)
 11. A method for deposition and patterning of highlyconductive ink on insulating PCB materials comprises the steps of: a.providing a single PCB insulating layer at a predetermined position; b.drilling a set of holes of predetermined positions and diameters in saidPCB insulating layer; c. forming a first trace layer onto said PCBinsulating layer; wherein said forming the first trace layer comprisesthe steps of depositing a first ink composition composed of a mixture ofa high melting point metallic powder, a low melting point metallicpowder, and a flux paste, and sintering said first ink composition; d.forming a second trace layer onto said first trace layer; wherein saidforming the second trace layer comprises the steps of depositing asecond ink composition, composed of a mixture of a low melting pointmetallic powder and a flux, and sintering said second ink composition;e. repeating steps a-d on each single PCB insulating layer.
 12. Themethod of claim 11 further comprising via patterning including blind,buried and through hole vias comprising steps of drilling via holes insingle PCB insulating layers, attaching said layers to already-processedstacks of such PCB layers, and adding conductive traces to said layersincluding filling of said holes with said conductive ink.
 13. The methodof claim 11 further comprising multilayer stacking consisting of steps:a. attaching a high temperature adhesive layer protected by a Mylar filmto a reverse side of each said single PCB insulating layer, drillingregistration holes, via holes, and cut out release cuts; b. aligningsaid single PCB insulating layer to printing system coordinates; c.removing said Mylar film before said single PCB insulating layer isattached to already-processed PCB layers below it in a stack.
 14. Themethod of claim 13 wherein said high temperature adhesive layer is a 3MVHB Adhesive Transfer Tape or prepreg material.
 15. A set ofcompositions for depositing two-layered conductive traces comprising afirst and second composition, said first composition being composed of amixture of a high melting point metallic powder, such as copper, silver,gold, and a low melting point metallic powder, such as zinc, tin, lead,alloys of such metals, plus a flux paste, either organic, inorganic, ora mixture thereof; the ratio of high melting point powder to low meltingpoint powder being between 5:1 and 1:5, while the flux content being10%-20% by weight; said second composition being a mixture of a lowmelting point metallic powder, such as zinc, tin, lead, alloys of suchmetals, and flux paste either organic, inorganic, or a mixture thereofat a ratio of 10%-20% by weight.
 16. An end-to-end system for depositionand patterning of highly conductive ink on insulating PCB materials asclaimed in claim 11 comprising PCB layer magazine, feeder, drillingstation, verification means, stacking station, alignment means,patterning station, and sintering station; wherein the patterningstation sequentially extrudes a plurality of conductive ink compositionsonto a top insulating layer.
 17. The system of claim 16 wherein saidconductive ink compositions comprise a first composition and a secondcomposition, said first composition being composed of a mixture of ahigh-melting-point metallic powder, a low-melting-point metallic powder,and flux paste, and said second composition being a mixture of a lowmelting point metallic powder and flux paste, the weight ratio of saidhigh melting point powder to low melting point powder being between theratios of 5:1 and 1:5, and the content of said flux paste being 10%-20%by weight.
 18. The system of claim 16 wherein said verification meanscomprises optical means.
 19. The system of claim 16 wherein saiddrilling station is selected from the group consisting of: mechanicaldrilling apparatus and laser drilling apparatus.
 20. The system of claim16 wherein said stacking station employs alignment means selected fromthe group consisting of: registration holes and optical system.
 21. Thesystem of claim 16, wherein said insulating PCB materials are selectedfrom glass-epoxy and polyimides.
 22. The method of claim 11, furthercomprising an attachment step after step b, wherein said attachment stepcomprises attaching said single PCB insulating layer at a predeterminedposition to those layers already processed in steps a-d by means of anadhesive layer.